Light-emitting sources with constrained dimensions have received much attention because of their importance in sensing and lab-on-a-chip applications. B. Choudhury, et al., J. J. Appl. Phys. 2004, 96 (5), 2949-2954; J. B. Edel et al., A. J. Lab Chip 2004, 4 (2), 136-140. The ability to shrink the size of an illumination source and couple it with on-chip detection mechanisms can enable the production of fully packaged, functional devices that operate without the need of external optics. This can, in turn, enhance the sensitivity and improve the signal-to-noise ratio of detected fluorescence by limiting the number of interfaces through which the emitted light has to propagate. Seo, J.; Lee, L. P. Sens. Actuators, B 2004, 99 (2-3), 615-622. However, most state of the art lab-on-a-chip devices use external illumination sources, such as lasers or light emitting diodes (LEDs). Even though LEDs are relatively inexpensive and can have submillimeter size, they are still much larger than the typical micro- or nanofluidic device and have to be mounted outside of the sensing region. Some attempts have been made at fabricating on-chip point sources that can be coupled to micro- and nanofluidic devices. However, these point sources are usually much larger than the fabricated devices, can be expensive to fabricate, or do not emit light in the visible spectrum. Light-emitting devices based on organic semiconductors have been demonstrated as efficient sources with characteristics that make them attractive for flat panel displays, lighting, and sensing applications. Holder, E.; Langeveld, B. M. W.; Schubert, U.S. Adv. Mater. 2005, 17 (9) 1109-1121. For many of these materials, the organic semiconductor can be deposited directly from solution, thus affording ease of fabrication. Within the class of solution processable organic electroluminescent materials, mixed conductors such as ionic transition metal complexes (“iTMCs”) have emerged as materials that allow the fabrication of efficient, single-layer light-emitting devices employing air stable electrodes. Bernards, D. A.; Flores-Torres, S.; Abruna, H. D.; Malliaras, G. G. Science 2006, 313 (5792), 1416-1419; Bernhard, S.; Barron, J. A.; Houston, P. L.; Abruna, H. D.; Ruglovksy, J. L.; Gao, X. C.; Malliaras, G. G. J. Am. Chem. Soc. 2002, 124 (45), 13624-13628. These features are a consequence of the operational mechanism of iTMC light-emitting devices, which is similar to that of polymer light-emitting electrochemical cells. In these materials, ionic space charge effects lead to both enhancement of electronic charge injection (enabling the use of air-stable electrodes) and a light emission profile confined to a thin region of the active area between the electrodes. These characteristics make organic light-emitting devices, and in particular iTMC devices, attractive candidates for on-chip light sources.
In this sense, having a light-emitting fiber based on an iTMC could provide a highly localized point source emission profile, with the axial dimension restricted by the iTMC operational mechanism and the radial dimension given by the diameter of the fiber. Such fibers also enable multiple highly localized point sources for lab-on-chip applications that existing formats do not allow. Moreover, such light-emitting fibers can be synthesized a simple electrospinning process. Electrospun light-emitting fibers based on iTMCs show distinct advantages: the emission zone is three-dimensionally confined to sub-wavelength dimensions; the emission dimensions are similar to the smallest light emitting sources produced to date (based on nanowires) but are produced using the far less involved and inexpensive fabrication techniques of electrospinning; the illuminated fibers have relatively long life-times; the brightness of light emission can be easily controlled by the applied voltage; and the color of the emission can be tuned by varying the iTMC complex used in the fiber.